At present, in transmission of electricity, the alternating current power transmission is usually carried out by using copper cables. Since this electric power transmission is accompanied with Joule loss due to electrical resistance, in view of an environmental problem such as CO2 emission and an energy-saving problem, it is desired that electric power transmission loss is made to be lowered, and that electric power cables are miniaturized.
In this viewpoint, in an electric power transmission cable in which a superconductive material is used, there are merits or advantages that the electric power transmission loss is small, and that a large capacity of electric power can be made to flow into the cable. Especially, in direct current power transmission, since electrical resistance is zero, it is possible to carry out the electric power transmission without the electric power transmission loss at all. In addition, according to the superconductive power transmission, since the large capacity of electric power can be transmitted by utilizing the forte of the superconductive power transmission, it is possible to omit conventional step-up and step-down transforming facilities to be installed near a power plant and a city, and thus the superconductive power transmission is economical.
As a superconductive power transmission method, there is a method using a high temperature superconductive material which can be easily cooled by liquefied nitrogen, and which features the fact that an electric current density is not so large, and such a method is put to practical use. Accordingly, in a future design of a superconductive cable, there remains a significant issue how large capacity of electric current an electrical power transmission can be carried out with.
Also, due to the fact that the superconductive material has a characteristic that there is no electrical resistance at a low temperature, it is expected that the superconductive material is applied to a variety of an energy-related engineering field, an electronics field, a medical field and so forth. In general, when an electric current is made to flow in a magnetic field, the Lorentz force acts on a quantized magnetic flux. Accordingly, when the magnetic flux is moved at a velocity of v due to that action, and when a magnetic flux density is defined as B, an induced electromotive force “E=B×v” is caused. Thus, normal conduction electrons are driven in the superconductive material so that electrical resistance is created similar to a case of a metal.
A critical current density, defined as the maximum current density at which an electric current can be made to flow into the superconductive material without the electrical resistance, is determined by a magnetic flux pinning mechanism (which has the function of stopping the movement of the magnetic flux against the Lorentz force acting on the interior of the magnetic flux to thereby prevent a production of an induced electric field), and a value of the magnetic flux is decreased with the increase in the magnetic field. In an electric power transmission cable, although an intensity of the magnetic field produced in the interior of the cable is not so large, it may reach a maximum of 0.5 T, especially when the electric power transmission is carried out at a large capacity of electric current, and thus the decline in the critical current density causes a large problem.
By contrast with this case where an angle between the magnetic field and the electric current is not 0° (hereinafter, the magnetic field concerned is referred to as an inclined magnetic field, especially it is referred to as a lateral magnetic field when the angle is 90°), in a case where the magnetic field and the electric current are parallel to each other (hereinafter, the magnetic field concerned is referred to as a longitudinal magnetic field), the Lorentz force does not act on the quantized magnetic flux, so that some strange phenomena can be observed. One of the strange phenomena is that the critical current density is considerably increased (hereinafter, this phenomenon is referred to as an longitudinal magnetic field effect). Usually, in the lateral magnetic field, the critical current density is decreased with the increase in the magnetic field. However, it is known that the critical current density is reversely increased in the longitudinal magnetic field. When the electric current and the magnetic field are parallel to each other in the longitudinal magnetic field, it is referred as a force-free state. In this case, when the current density is defined as J, it is known that J×B=0 is established. In this state, the magnetic flux has a strain (i.e., a force-free strain) as if a folding fan is opened, as shown in FIG. 10.
In general, in a case of a metal-based superconductive material, when an alternating current flows into a superconductive wire, it is a significant requirement that an alternating current loss is decreased. If the superconductive wire is divided into fine wires which are independent from each other, it is possible to decrease a hysteresis loss from which the alternating current loss is derived. Nevertheless, in reality, in order to take precaution against magnetic instability, the fine wires are electromagnetically bonded to each other by intervening a stabilization layer therebetween, which is composed of a normal conduction metal, so that the electric currents are bypassed around the stabilization layer. Thus, there are not merits or advantages which should be obtained from the division of the superconductive wire into the fine wires cables, so that the alternating current loss cannot be substantially decreased. This is because the self magnetic fields (i.e., the magnetic fields, produced in the circumferential direction) produced by the electric currents, penetrate into only the surface, so that no electric currents flow into the interior which the self magnetic fields do not reach.
In order to solve the aforesaid problem, a technique, in which a multi-core wire is twisted in the circumferential direction, is developed, with multi-core wire being produced by embedding the fine superconductive wires in a metal material such as copper or the like. Thus, the magnetic field in the circumferential direction can easily penetrate into the interior so that the electric currents further flow into the inner fine superconductive wires, whereby it is possible to decrease the alternating current loss. In this case, the alternating current loss is in proportion to the twist pitch.
In a case of an electric power transmission cable which needs a large capacity of electric current, since it is insufficient that the cable is constructed by only one superconductive wire, it is usual to produce a superconductive material by bundling some superconductive wires. In this case, due to a production method of the superconductive material, the individual superconductive wires are twisted in the same direction, and have the same pitch. Nevertheless, with this arrangement, the alternating current loss in the longitudinal magnetic field caused by the twist of the superconductive wires cannot be ignored though it has been ignored ever. Thus, it is usual that the individual superconductive wires are twisted in a reverse direction to a direction in which individual blank wires are twisted so that the inner longitudinal magnetic field component can be minimized.
In the case of the high temperature superconductive material, it is formed into a tape shape due to the crystallization structure and the peculiarity of the electromagnetic characteristic of the high temperature superconductive material. Thus, although the high temperature superconductive material is constructed into a multi-core structure, at the electromagnetic bonding of the multi-core structure is strong under the present conditions, and thus merits or advantages based on the multi-core structure is not so expected in a commercial network frequency.
Accordingly, in order to obtain the merits or advantages of the twist, the superconductive material is constructed by gathering together a plurality of superconductive wires, and the individual inner superconductive wires are twisted in a reverse direction to a direction in which the individual outer superconductive wires so that the magnetic field in the longitudinal direction becomes as small as possible.
In the present conditions as mentioned above, for example, techniques concerning superconductive cables are disclosed in Patent Documents 1 to 7. The technique disclosed in Patent Document 1 is directed to an alternating current superconductive cable in which individual twisted angles of conductive layers are increased or decreased stepwise between a twisted angle of the radially innermost conductive layer and a twisted angle of the radially outermost conductive layer. The technique disclosed in Patent Document 2 is directed to a calculation method to calculate twisted angles for reducing losses in a similar alternating current superconductive cable.
The technique disclosed in Patent Document 3 is directed to a multi-layered superconductive cable, having a plurality of superconductive layers concentrically combined with each other, which cable is defined as either a coaxial bi-directional cable having more than three forward current path layers or conductive layers or a united-type three-phase cable having a shield layer, wherein the cable is manufactured so that a winding pitch angle of each of the layers is represented by a cubic equation based on a core radius of a standardized layer, whereby inductance values of the individual layers are uniformed to thereby increase critical current densities.
The technique disclosed in Patent Document 4 is directed to a superconductive cable including two kinds of cable cores (i.e., a first cable core and a second cable core), having different structures, which are twisted and combined with each other, and a thermal insulating pipe in which the twisted cable cores are received. The first cable core includes a first superconductive layer which is used as a forward current path or a polar power transmission wire in direct current power transmission, and no superconductive layer except for the first superconductive layer. The second cable core includes a second superconductive layer which is used as a backward current path or a neutral wire, and no superconductive except for the second superconductive layer. The second superconductive layer has an inside diameter which is larger than an outside diameter of the first superconductive layer.
The technique disclosed in Patent Document 5 is directed to a superconductive cable including two cable cores which are twisted and combined with each other, with each of the cables being composed of a superconductive conductor layer and an outer superconductor layer which are formed of superconductive material, and a thermal insulating pipe in which the twisted cables are received. Each of the cable cores has a former, the superconductive conductor layer, an insulating layer, the outer superconductor layer and a protective layer which are arranged in order from the center thereof. In unipolar electric power transmission, each of the superconductive conductor layers included in both the cables is used as a forward current path into which a unipolar electric current flows, and each of the outer superconductor layers included in both the cables is used as a backward current path into which a backward electric current flows. In bipolar electric power transmission, the superconductive conductor layer included in one cable is used for positive power transmission, and the superconductive conductor layer included in the other cable is used for negative power transmission, with each of the outer superconductor layers included in both the cables being used as a neutral wire layer.
The technique disclosed in Patent Document 6 is directed to a superconductive cable having a plurality of high temperature oxide superconductors which are arranged in the same direction so as to be adjacent to each other, wherein electric power transmission is carried out so that respective electric currents flow into the two adjacent high temperature oxide superconductors in reverse directions to each other.
The technique disclosed in Patent Document 7 is directed to a transitional superconductive tape unit in which an even number of tape-like superconductors are transitionally twisted and combined with each other, and a piece of superconductor application equipment using the transitional superconductive tape unit, and a superconductive cable wherein the transitional superconductive tape unit is wound around a cylindrical pipe member.